Electronic elements that employ a carbon nanotube (CNT) have been the subject of intense research. Carbon nanotubes are based on a structure obtained by rolling a graphene sheet with a hexagonal mesh-like structure of carbon atoms into a cylindrical shape, and single-wall carbon nanotubes and multi-wall carbon nanotubes exist. Carbon nanotubes can have a wide bandgap in the same way as semiconductors and can be an electron material that can be utilized in semiconductor devices. Although still at a basic research stage, a variety of application examples for electronic devices with carbon nanotubes have been reported.
For example, WO 02/063693A1 proposes a structure of electrodes electrically connected to a multi-wall carbon nanotube. Accordingly, the carbon nanotube is cut directly before forming the electrodes and metal with a strong chemical bond with carbon atoms is formed on a carbon nanotube obtained by cutting to form the electrodes. As a result, applications to electronic devices are attempted by reducing the contact resistance between the electrodes and the carbon nanotube.
In addition, Japanese Patent Laid-open Publication No. 2004-171903 proposes a field effect transistor in which a metallic inner wall of a double wall carbon nanotube is the gate electrode and a semiconductive outer wall is the channel. This Patent Document also discloses, as prior art, a field effect transistor in which a semiconductive inner wall of a double wall carbon nanotube is the channel region and a metallic outer wall is the gate electrode.
Generally, a field effect transistor having a carbon nanotube as the channel region possesses superior characteristics such as a long ballistic conduction length over which carriers move without scattering, a high-speed switching capability, and an improved high-frequency response characteristic.
However, because the diameter of a carbon nanotube is on the order of nm and extremely small, there is the problem that the innermost wall is readily affected by surface traps that are formed at the surface of the outermost wall. That is, the electrical conduction of the carbon nanotube is affected and the IV characteristic of the transistor has noise superposed thereon in accordance with the repetitive motion of the electrons between the traps formed by impurities that adhere to the surface of the outermost wall and the carbon nanotubes. Although forming a passivation film on the surface of the carbon nanotube may be considered, traps in the passivation film or at the interface between the film and the carbon nanotube cannot be completely dispensed with. The existence of such traps causes the loss of the superiority of the high-speed switching characteristic, the high-frequency characteristic and the low noise characteristic of a transistor that utilizes a carbon nanotube.